1. Field of the Invention
This invention relates to devices for detecting emergency vehicles as such and providing means of traffic signal alignment along a city street or roadway in advance of such vehicles to stop all cross traffic and give right-of-way to these vehicles so as to provide rapid, unimpeded movement of emergency vehicles through a city or urban area.
2. Description of the Prior Art
There are several methods of preemption of signals for emergency vehicles in use at present. The most simple of which is a push button installed at fire stations to preempt traffic signals immediately adjacent to the fire station when fire trucks leave on emergency calls. The button is pushed just prior to fire trucks leaving the station. Traffic signals immediately adjacent to the station are preempted to provide right-of-way through these signals for the trucks. A preadjusted timer returns the signal controller to normal operation. This type of preemption is limited to vehicles leaving the fire station and to traffic signals immediately adjacent to the home base of the vehicles. Once the vehicles have passed through these preempted signals, there is no further preemption ahead and the vehicles are forced to weave through traffic, travel on the wrong side of the street and go through red lights hoping cross traffic will heed the siren's wale and flashing red beacon mounted on the vehicle.
A second type of emergency vehicle preemption system utilizes a modulated light source mounted on top of emergency vehicles with a sensor that detects the modulated light mounted on signal standards or a special mounting near the intersection. The circuitry for these systems is complicated, relatively expensive and require considerable maintenance to keep operative. Any obstacle which blocks the light beam such as large trucks in the path can render the system inoperative.
A third type of signal preemption system utilizes portable low power, radar with the transmitter mounted on the emergency vehicle and the receiver located at the signalized intersection. These systems suffer essentially the same problems as modulated light systems.